1. Field of the Invention
The invention relates to a method for recognizing erratic combustion in multicylinder internal combustion engines. In engines equipped with a catalytic converter, erratic combustion can cause damage to the catalytic converter, since after-reactions of the uncombusted mixture of fuel and air can cause high temperatures in the catalytic converter. Regardless of whether or not a catalytic converter is used, erratic combustion causes poorer-quality exhaust.
Methods are already known that measure the instantaneous angular speed of the crankshaft in order to detect erratic combustion. The period of time during which the crankshaft rotates about a defined angle is measured. The time measurement is typically carried out with the aid of markings on a wheel mounted on a crankshaft. The difference between successive periods of time that are measured is compared with the limit value. Erratic combustion leads to a temporary slowing of the angular speed of the crankshaft, since the energy that would be contributed by the misfiring cylinder is missing in the drive of the crankshaft. If the angular speed slows down, the difference between successive measured time periods increases. If a predeterminable limit value is exceeded, an ignition misfire is recognized and indicated or displayed. Control provisions can then optionally be made, with one example being to turn off the applicable injection nozzle.
A primary disadvantage of the known methods is that they are suitable only for steady-state operation at a constant speed, without braking and acceleration. In braking, for instance, the angular speed of the crankshaft decreases. The measured time period and the difference between successive time periods become greater, and as soon as that increase in the time period exceeds a limit value, a combustion misfire is indicated. Yet no misfire has in fact yet occurred, rather simply entirely normal braking took place. German Published, Non-Prosecuted Application DE 40 09 895 A1 describes a method that overcomes that disadvantage.
In the method described in German Published, Non-Prosecuted Application DE 40 09 895 A1, not only a static component but also a dynamic component is calculated, which takes into account the mean linear increase in speed (acceleration) or the mean linear decrease in speed (deceleration). The dynamic component is calculated so that periods of time of a plurality of cylinders that are successive, but spaced farther apart timewise, are compared with one another. If the dynamic component is then subtracted from the static component, that compensates for the influence of changes in speed on the measured time periods. The remaining changes in the time periods are then in fact predominantly due to erratic combustion.
A disadvantage of the method described above is that once again it is unsuitable for a markedly unsteady-state operation, because it can compensate only for the influences of constant acceleration or constant deceleration. Yet those driving states tend to be the exception in everyday operation of a motor vehicle. Under everyday conditions, usually highly unsteady conditions prevail instead, with examples being uneven deceleration, uneven acceleration, an often abrupt change back and forth between acceleration and deceleration, or even very hard changes in speed, such as in fast clutch engagement and disengagement.